1. Technical Field
The invention relates to control of speed ratio upshifts for a vehicle automatic transmission characterized by reduced transmission output shaft torque transients during an upshift.
2. Background
Known automatic transmissions for automotive vehicles include step ratio controls for effecting speed ratio changes in response to changing driving conditions. The term “speed ratio”, for purposes of this description, is defined as transmission input shaft speed divided by transmission output shaft speed.
A so-called speed ratio upshift occurs when the driving conditions require a ratio change from a so-called low ratio (high speed ratio) to a so-called high ratio (low speed ratio) in the transmission gearing. The gearing may include, for example, either a planetary type gear system or a lay shaft type gear system. An automatic gear ratio shift is achieved by friction torque establishing devices, such as multiple disk clutches and multiple disk brakes. The friction torque establishing devices include friction elements, such as multiple plate clutches and band brakes, which may be actuated hydraulically or mechanically. One friction element is engaged in synchronism with disengagement of a companion friction element. However, for purposes of this description, the friction elements may be referred to as an on-coming coupling, clutch or brake and an off-going coupling, clutch or brake. The upshift event is characterized by a preparatory phase, a torque phase and an inertia phase as the vehicle accelerates from a standing start.
During the preparatory stage, a transmission controller reduces off-going clutch torque capacity to prepare for its release. Simultaneously, it adjusts the position of an on-coming clutch actuator to prepare for its engagement. During the torque phase, the controller increases on-coming clutch torque capacity. This causes torque that is transmitted through the off-going clutch to drop significantly in accordance with an increase in torque capacity of the on-coming friction element. The controller may maintain enough off-going clutch torque capacity to keep the off-going clutch securely engaged or locked during the torque phase, which immediately follows the preparatory phase. Alternatively, the controller may allow the off-going clutch to slip at a controlled rate.
During the torque phase of a conventional control system, torque transmitted through the off-going clutch decreases when the transmission output shaft torque drops. This creates a so-called torque hole. A large torque hole can be perceived by the vehicle occupants as an unpleasant shift shock. The inertia phase begins when the off-going clutch is released with no significant torque capacity.